1. Field of the Invention
This invention relates to a wireless communications and, more particularly, to a receiver architecture in a wireless communications system using analog to digital conversion at radio frequency (RF).
2. Description of Related Art
The service area of a wireless communications system is partitioned into connected service domains known as cells, where wireless units communicate via radio links with a base station (BS) serving the cell. The base station is coupled to a land network, for example through a Mobile Switching Center (MSC) which is connected to a plurality of base stations dispersed throughout the service area. In the wireless communications industry, a service provider is often granted two or more non-contiguous or segregated frequency bands to be used for the wireless transmission and reception of RF communications channels. For example, in the United States, a base station for an xe2x80x9cAxe2x80x9d band provider for cellular communications receives frequency channels within the A (825-835 MHz), Axe2x80x2 (845-846.5 MHz) and Axe2x80x3 (824-825 MHz) bands, and the wireless units receive frequency channels within the A (870-880 MHz), Axe2x80x2 (890-891.5 MHz) and Axe2x80x3 (869-870 MHz) bands. A base station for a B band provider receives frequency channels within the B (835-845 MHz) and Bxe2x80x2 (846.5-849 MHz) frequency bands, and the wireless units receive frequency channels within the B (880-890 MHz) and Bxe2x80x2 (891.5-894 MHz) frequency bands. Additionally, a base station for a Personal Communications Systems (PCS) provider may receive frequency channels from wireless units on one or more PCS bands (1850 MHz-1910 MHz), and the wireless units receive frequency channels on one or more PCS bands (1930-1990 MHz).
In order to reduce system hardware costs, a service provider would want to use a common receiver for the simultaneous reception and processing of signals within the non-contiguous frequency bands. In a typical receiver architecture, a down-conversion stage for each frequency band is typically used to down-convert and to manipulate the placement of each frequency band at intermediate frequencies (IF) such that the frequency bands of the modulated analog signals are converted to a corresponding IF frequency spectrum and can be sampled at a reduced sampling rate by separate analog to digital (A/D) converters. To use a single A/D converter to digitize the modulated analog signals in the non-contiguous bands, a single A/D would have to sample at a high enough rate to encompass both frequency bands. This is an inefficient approach because the A/D converter is using bandwidth in sampling unwanted frequencies in the gap between the frequency bands. To reduce the frequency gap between non-contiguous frequency bands, a down-conversion stage for each of the frequency bands is used to down-convert and manipulate the placement of each frequency band at IF such that the bands are closer together to fit in a smaller Nyquist bandwidth. Another approach to improve the efficient use of the A/D converter bandwidth involves down-converting both frequency bands such that a replica of one of the frequency bands is positioned in the frequency gap between the frequency bands.
When the IF spectrum is sampled by an A/D converter at a sampling rate which is greater than or equal to twice the combined signal bandwidth, which can be referred to a the Nyquist sampling rate, the A/D input signal bandwidth rotates or folds periodically about itself at multiples of one-half the sampling frequency. As such, the signal bandwidth and mirror images of the signal bandwidth are periodically repeated at frequency intervals corresponding to the sampling rate of the A/D converter. Each replica of the signal bandwidth can be referred to as a Nyquist zone, and the IF signal bandwidth folds back to the first Nyquist zone between about 0 Hz and one-half the sampling frequency. The bandwidth of a Nyquist zone corresponds to the Nyquist bandwidth.
The periodicity of the spectral density in the digital domain is a basic property of sampled waveforms which can be predicted by determining the Fourier transform of the time-sampled waveform. Generally, the A/D converter samples at at least twice the bandwidth of the composite frequency bands (i.e. the Nyquist sampling rate) to obtain a digital representation of the modulated analog IF signal. Accordingly, the sampling rate for the A/D converter is chosen such that the Nyquist bandwidth encompasses the desired IF frequency bands. The higher the sampling rate, the wider is the Nyquist bandwidth. If the waveform is sampled at a rate less than twice its signal bandwidth (the Nyquist bandwidth), an undesirable overlapping between the adjacent periodic spectrums can occurxe2x80x94a well known phenomena known as aliasing. Accordingly, the sampling rate and the IF frequency are chosen such that the Nyquist bandwidth encompasses the frequency band to be converted while reducing the sampling rate of the A/D converter, enabling the use of lower sampling rate A/D converters with reduced cost. Accordingly, the wider the separation or frequency gap between the frequency bands, the current receiver architectures reach a point where the use of a single A/D is not viewed as practical or efficient.
If the frequency bands are far enough apart or if desired, a separate antenna is used for each segregated frequency band. In multiple antenna architectures where antennas are dedicated to different frequency bands, a separate branch is used in which a frequency conversion stage including a mixer and a local oscillator (LO) are used to down-convert the radio frequency (RF) analog signals to intermediate frequencies (IF), and an A/D is typically used for each antenna path. The above receiver architectures do not take advantage of the potential bandwidths and flexibility provided by the A/D converters in converting analog signals into the digital domain.
The present invention involves a receiver which receives analog signals at radio frequency (RF), and the RF analog signals are converted into the digital domain. As such, the receiver does not require frequency conversion stage(s) prior to analog to digital conversion. For example, the receiver can comprise at least one antenna(s) which receives radio frequency (RF) analog signals at different frequency bands. The analog RF signals are provided to a single analog to digital (A/D) converter, and the A/D converter converts the analog RF signals at the different frequency bands into digital signals within the Nyquist bandwidth. By properly selecting the sampling rate of the A/D converter for the RF analog signals, the A/D converter can produce replicas of the different frequency bands of the analog signals in non-overlapping portions of the Nyquist bandwidth.